Field
Aspects of the present invention generally relate to a sheet processing apparatus and an image forming apparatus having a binding processing function.
Description of the Related Art
A stapling device has conventionally been used widely as a device for binding sheets on which images are formed by an image forming apparatus such as a copying machine and a printer. The stapling device performs binding processing to bind a sheet bundle including a plurality of sheets by using a binding member such as metal staples. However, when using each sheet of the sheet bundle stapled by the stapling device as a document to be read, the staples binding the sheet bundle need to be removed. When recycling the sheet bundle bound by staples, the staples binding the sheet bundle also need to be removed to separately collect the sheets and the staples from the viewpoint of environmental protection. Since the staples used for the binding processing are discarded after being used, there has been a problem in terms of reuse of resources.
Japanese Patent Application Laid-Open No. 2004-155537 discusses a sheet binding device that uses no binding member such as a staple to reduce time and effort when reusing the sheets as a document or at the time of recycling. Using no staples, such a sheet binding device discards no staples. The sheet binding device is configured to, after a plurality of sheets conveyed from an image forming apparatus is bundled and aligned into a sheet bundle, press against sheets a tooth die having protrusions and recesses for forming recesses and protrusions in part of the sheet bundle. The sheet binding device performs binding processing by thus pressing the sheet bundle to entangle fibers of the sheet bundle with each other.
In a case where the conventional stapleless binding method described above is applied to an image forming apparatus, it is conceivable that an actuator is used as a driving source for pressing the tooth die having protrusions and recesses against the sheet bundle to automate the pressing operation. In the stapleless binding processing, steady application of constant pressing force to the sheet bundle is important in maintaining the quality of the sheet bundle after undergoing the binding processing so that the retention force of the binding portion lasts and the bound portion will not get broken. In order for the actuator to provide constant pressing force, the output torque of the actuator can be controlled by controlling the driving current value received by the actuator to be a predetermined value. The predetermined value is selected to be smaller than a value of the driving current corresponding to maximum output torque that the actuator can output. The reason is that the pressing force needed for the binding processing has a predetermined range that differs depending on the number and a type of sheets of the sheet bundle.
If the pressing force needed to be applied to the sheet bundle is low, the actuator is controlled by a driving current value lower than usual throughout the binding processing operation. In such a case, the output torque that the actuator can produce at start-up is also limited to a low value similar to the binding processing operation. This increases the time needed for the start-up of the actuator and increases the time of the entire binding processing operation. Accordingly, since the time needed for the stapleless binding operation increases, there is a problem that the mounted sheet processing apparatus and/or the overall productivity of image forming apparatus decreases.
The thickness of the sheet bundle and the density of sheets vary according to the number of sheets and paper type of the sheet bundle. As a result, the timing at which constant pressing force is applied to the sheet bundle varies. If the period of application of the constant pressing force to the sheet bundle is not properly adjusted, a phenomenon in which the sheet bundle exfoliates easily (hereinafter, referred to as poor binding) can occur. Application of excessive pressure to the sheet bundle can break the sheets.